Multifaceted Roles of Alkyltransferase and Related Proteins in DNA Repair, DNA Damage, Resistance to Chemotherapy, and Research Tools

Abstract

O6-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O6-position of guanine from DNA restoring the DNA in one step. The protein acts only once and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not alone act to repair O6-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA crosslinks is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents and the availability of AGT inhibitors such as O6-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally and synthetic variants engineered to react specifically with derivatives of O6-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.

A Novel Temozolomide–Perillyl Alcohol Conjugate Exhibits Superior Activity against Breast Cancer Cells In Vitro and Intracranial Triple-Negative Tumor Growth In Vivo

“…For instance, we show that the presence of MGMT, which highly specifically repairs O6-methylguanine and provides profound protection against TMZ (10,11), minimizes DNA damage caused by T-P (Fig. 5D) and increases cellular resistance to this agent (Fig.…”

“…However, when TMZ was tested for activity against brain metastatic breast cancer in heavily pretreated patients, it revealed mixed outcomes that ranged from "encouraging activity" and "disease control" to "well-tolerated, but no objective responses" (4)(5)(6)(7)(8)(9). The underlying basis for these inconsistent results was not investigated, but it is conceivable that these differences may have been because of variable expression levels of O6-methylguanine-DNA methyltransferase (MGMT; also called O6-alkylguanine-DNA alkyltransferase, AGT), a DNA repair enzyme that removes alkyl groups located on the O6-position of guanine (10,11). Because the primary toxic DNA lesion set by TMZ is alkylation of O6-guanine, high expression levels of MGMT protect tumor cells from the cytotoxic impact of TMZ and provide treatment resistance (12,13).…”

“…MGMT activity is unusual in that it represents a "suicide" mechanism, whereby acceptance of the alkyl group from DNA irreversibly inactivates the enzyme and leads to its rapid degradation (10). This feature is exploited by the use of specific MGMT inhibitors, such as O6-benzylguanine (O6-BG), which act as pseudosubstrates (15).…”

Biochemical and Structural Studies of the Mycobacterium tuberculosis O 6 -Methylguanine Methyltransferase and Mutated Variants

“…The majority of living organisms attain alkylatedbase repair by deploying different strategies, depending upon the chemical nature of the alkyl group, the entity of the damage, and the physiological condition of the cell: (i) the multistep excision of a short, lesion-containing strand followed by new DNA synthesis; (ii) the substitution of the modified base in toto; or (iii) the direct surgical removal of the alkyl-substituting group from the base by sacrificing one molecule of a DNAprotein alkyltransferase (8,9), such as the O 6 -methylguanine-DNA methyltransferase (OGT) (EC 2.1.1.63). All OGT members studied until now invariably act through a suicidal mechanism (10), by performing the stoichiometric transfer of the O 6 -alkyl group from the modified guanine to a strictly conserved cysteine residue in the protein active site, which is hosted in the C-terminal domain of the protein. This covalent modification leaves OGT permanently inactivated and possibly more prone to degradation (11,12) (Fig.…”

Alkyltransferase-like protein (Atl1) distinguishes alkylated guanines for DNA repair using cation–π interactions

“…O 6 -Alkylguanine-DNA alkyltransferases (AGTs) repair many different O 6 -alkylguanine lesions by transferring the alkyl group to an active site Cys (1)(2)(3)(4), although some are reported to be poorer substrates (5)(6)(7)(8)(9)(10). Alkyltransferase-like (ATL) proteins (for reviews see refs.…”